News Stories
January 2010
CE NEWS- Wall Art, Engineered Solutions for Aesthetic Earth Retention
By Bob Drake
Art and science truly combine in today’s retaining wall systems. Civil engineers must design increasingly taller structures that not only support slopes, buildings, or roads, but also enhance the aesthetics of the project. The retaining wall can be a work of art itself, making an architectural statement; or it may disappear into the natural elements surrounding it, whether vegetation, soil, or rock. According to manufactures, the most significant trends influencing the use of segmental, mechanically stabilized earth (MSE), gravity, and geocellular retaining walls include new project delivery and design methods, a focus on green or sustainable design, and wall aesthetics.
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June 2007
Construction Executive Magazine: What Makes A Great Wall?
By David McKittrick
If ever there was a testimonial about the integrity of a wall, it would have to come from the builders of the Great Wall of China. Even though some original sections have surrendered to natural and man-made forces, it is still standing more than 2,300 years after construction began. While the construction material of choice for today’s retaining structures is conceptually similar to that of the Great Wall, the application of technology gives modern walls a higher measure of strength, durability, and safety.
Recognized as one of the most important civil engineering innovations of the 20th century, mechanically stabilized earth (MSE) technology allows retaining structures to be designed and built higher, stronger, and more economically than ever before. Thirty years after its introduction in the U.S., the most popular MSE configuration uses steel strips embedded in granular fills and faced with precast concrete panels, even though many variations of soil reinforcements and facing systems are available.
Their safety and reliability features have made these MSE structures the retention system of choice for many state departments of transportation (DOTs) that are responsible for public safety along roadways. More than half of DOT-deployed retaining structures are MSE-based, which amounts to roughly 10 million square feet a year. Their success here is grounded in the design/build concept where structure performance, rather than end product or materials, is specified in contracts. DOT contractors have the flexibility to use whatever materials and construction methods give the best value for the required level of performance. For commercial developers, a similar approach also makes sense when they adopt MSE technology to re-grade and make sub-prime sites economically viable.
Taking a page from the DOT playbook, here are a few performance specifications to consider:
1. Design Responsibility. Developers can lower risk when their specification requires the general contractor to subcontract MSE design and construction to a specialist with relevant experience and appropriate levels of insurance. In consultation with the engineer of record, the developer needs to specify which party will be responsible for the structure’s internal and global stability.
2. Design Standards. Others are available, but the most comprehensive design standard comes from the American Association of State Highway and Transportation Officials (AASHTO). In addition to ensuring design quality, compliance with AASHTO has an added benefit if supporting site roads will be adopted by state or local governments.
3. Surcharges and external loading conditions. External conditions generally determine the required size of the structure, so it is critical to specify these in clear terms.
4. Structure Envelope. Specify the structural envelope to ensure the necessary amount of right of way, and any minimum or maximum batters (the back-leaning angle that’s built into the front face of a structure). Keep in mind that a 70-degree stepped wall has more batter and is less expensive to build than one with a vertical face, but takes up more land.
5. Settlement. Depending on the variability in the geometry and ground conditions, make sure the specifications include reasonable settlement criteria are compatible with other aspects of the project such as abutting structures and services. If staged construction is necessary, specify it.
6. Aesthetics. Stipulate the desired look, such as concrete for architectural treatment or gabion or grassed facings to achieve a more natural rustic look.
By specifying performance, rather than materials or end-products, developers can take advantage of several innovations in MSE construction that save time and money. For example, MSE walls incorporating geosynthetic straps – developed for the chloride-rich soils of the Middle East – are becoming more common as a substitute for steel. The polymer-based construction of the geosynthetic straps provides a higher level of durability so they can be used with recycled concrete, which costs significantly less in geographic areas where quarried backfills are expensive (this may also help qualify a project for credits under the Leadership in Energy and Environmental Design rating system of the Green Building Council). Another example is the increased use of welded wire facings and “green” walls that accommodate high settlements and cost less than concrete or stone-faced structures.
With their recent refinements MSE structures are an increasingly important part of today’s commercial development environment. Their performance specifications written in the context of a design/build contract gives developers a comfort level with appropriate tradeoffs and the best value, so they too can build a Great Wall.
